The present invention relates to cryogenic refrigerators, in particular, GM type pulse tube refrigerators. This type of refrigerator is comprised of a compressor unit that is connected by gas lines to an expander unit, commonly referred to as a cold-head. Two-stage GM type pulse tubes running at low speed are typically used for applications below about 20 K. It has been found that best performance at 4 K has been obtained with the pulse tube shown in FIG. 9 of U.S. Pat. No. 6,256,998. This design has six valves which open and close in the sequence shown in FIG. 11 of that patent. It contains a good description of the mechanism for producing refrigeration at very low temperatures and the important role that the valves and buffer volumes play in shaping the P-V diagram, FIG. 10, to maximize the refrigeration produced per cycle. The relationship between the displacement of the gas and the cycling of the pressure that is seen in the P-V diagram is usually referred to as phase shifting.
The first pulse tube refrigerator was described by W. E. Gifford in U.S. Pat. No. 3,237,421. A significant improvement in performance was made by E. I. Mikulin, A. A. Tarasow and M. P. Shkrebyonock, ‘Low temperature expansion (orifice type) pulse tube’, Advances in Cryogenic Engineering, Vol. 29, 1984, p. 629-637 in 1984, when they added an orifice and buffer volume to the warm end of the pulse tube. This improved the phase shifting. S. Zhu and P. Wu, ‘Double inlet pulse tube refrigerators: an important improvement’, Cryogenics, vol. 30, 1990, p. 514, made a further improvement by adding a second orifice between the warm end of the pulse tube and the inlet to the regenerator. The application of the “double-orifice” principal to a two-stage pulse tube enabled the production of refrigeration below 4 K. Production of refrigeration below 4 K has also been achieved by means of a “four-valve” pulse tube as described in U.S. Pat. No. 5,335,505 and U.S. Pat. No. 5,412,952. The first patent describes single stage pulse tubes and the second describes two-stage pulse tubes. Phase shifting is achieved by the timing of opening and closing the valves between the warm ends of the pulse tubes and the lines to and from the compressor relative to the timing of the valves between the warm end of the regenerator and the lines to and from the compressor. U.S. Pat. No. 5,412,952 describes the addition of buffer volumes connected to the warm ends of the two pulse tubes, with fixed orifices in each connecting line.
The valve timing described in U.S. Pat. No. 6,256,998 has been found to be more effective than the timing described in U.S. Pat. No. 5,412,952. U.S. Pat. No. 6,256,998 refers to the combination of “four-valve” control, with buffers and fixed orifices, as a “hybrid” pulse tube. It is to be noted that a “fixed” orifice may be adjustable, so that it can be set in a fixed position when the pulse tube is manufactured.
The four-valve pulse tube is more compact than a double-orifice pulse tube because it does not have buffer volumes; however it is less efficient because all of the phase shifting flow comes from the compressor. Most of the phase shifting flow in the double-orifice pulse tube is exchanged between the buffer volumes and the pulse tubes. A hybrid pulse tube is more efficient than a four-valve pulse tube and can have smaller buffer volumes than a double-orifice pulse tube. The buffer volumes can range in size from almost the zero volume of a four-valve pulse tube to almost the same size as a double-orifice pulse tube would have. Buffer volumes are defined relative to the corresponding pulse tube volumes as a ratio. One of the objects of this invention is to define the volumes for first-stage and second-stage buffers of a two-stage hybrid pulse tube that have been found to provide a good balance between efficiency and size. Another object of this invention is to integrate the buffer volumes into the warm end housing of the pulse tube cold-head assembly.
Some of the concepts that are incorporated in current pulse tube cold-heads are shown in U.S. Pat. No. 3,620,029. This is an early pneumatically driven GM type expander in which the motion of the displacer is controlled by gas flowing through a fixed orifice between the top of the displacer cylinder and a buffer volume. FIG. 7 of this patent shows a valve motor, in a separate housing, driving a rotary valve, that cycles flow at high pressure from a compressor to a regenerator, then returns it from the regenerator to the compressor at low pressure. The buffer volume is shown in a housing that is separate from the valve motor housing and the valve disc housing. If the displacer were to be removed from the cylinder, the cold-head would become a single orifice pulse tube. In actual practice the buffer volume has been integrated into the valve disc housing as shown in FIG. 1A of U.S. Pat. No. 6,256,997. Many pulse tube patents show the buffer volumes as being separated from the cold head by connecting tubes, e.g. U.S. Pat. No. 5,107,683, U.S. Pat. No. 5,295,355, U.S. Pat. No. 6,256,998, U.S. Pat. No. 6,343,475 and U.S. Pat. No. 6,434,947.
U.S. Pat. No. 6,378,312 describes a means of integrating one or more buffer volumes, orifices, a valve mechanism, and a valve motor within an integral housing; a housing which has several machined chambers. The pressure oscillation controller, which is usually a rotary valve disk in contact with a valve seat, has to be replaced or repaired when maintenance is needed. This integral configuration has the disadvantage of more difficult maintenance than a cold-head with more direct access to a rotary valve disc.